core: move cross-spawn-spawner from opencode to core package

Moved the cross-spawn-spawner module from packages/opencode to packages/core
to enable code sharing across the monorepo. This consolidates the process
spawning infrastructure into the core package so other packages can use
cross-platform child process spawning without duplicating the implementation.

Updated all import statements across the codebase to reference the new
location (@opencode-ai/core/effect/cross-spawn-spawner). Removed the
local copy from the opencode package along with its tests.

packages/core/package.json

@@ -18,23 +18,21 @@
   "imports": {},
   "devDependencies": {
     "@tsconfig/bun": "catalog:",
-    "@types/semver": "catalog:",
     "@types/bun": "catalog:",
-    "@types/npmcli__arborist": "6.3.3"
+    "@types/cross-spawn": "catalog:"
   },
   "dependencies": {
     "@effect/opentelemetry": "catalog:",
     "@effect/platform-node": "catalog:",
-    "@npmcli/arborist": "catalog:",
     "@opentelemetry/api": "1.9.0",
     "@opentelemetry/context-async-hooks": "2.6.1",
     "@opentelemetry/exporter-trace-otlp-http": "0.214.0",
     "@opentelemetry/sdk-trace-base": "2.6.1",
     "effect": "catalog:",
+    "cross-spawn": "catalog:",
     "glob": "13.0.5",
     "mime-types": "3.0.2",
     "minimatch": "10.2.5",
-    "semver": "catalog:",
     "xdg-basedir": "5.1.0",
     "zod": "catalog:"
   },